Water heater glossary

First hour rating (FHR) — how many gallons of hot water a storage unit can deliver in the first hour starting fully heated. This is the real capacity number, not tank gallons. ENERGY STAR requires at least 51 gallons FHR for certified gas storage units and at least 45 for certified heat pump units.

UEF (Uniform Energy Factor) — the standardized efficiency rating. Higher is better within the same type. ENERGY STAR 2026 criteria require at least 0.81-0.86 for gas storage, at least 0.95 for gas tankless, and at least 3.30 for integrated heat pump units. GPM — gallons per minute, the flow rating used for tankless units, only meaningful when paired with a temperature rise (ENERGY STAR uses a 67°F rise as the reference).

Anode rod — a sacrificial metal rod inside the tank that corrodes instead of the steel. Dip tube — the pipe that delivers incoming cold water to the bottom of the tank. Sediment — minerals settled on the tank floor; the reason for annual flushing. Expansion tank — a small vessel that absorbs pressure from heated water expanding in a closed system; often required by local code.

T&P valve — temperature and pressure relief valve, the safety device that prevents dangerous pressure buildup. Never cap or plug it. Atmospheric, power vent, and direct vent — three different ways a gas unit gets combustion air and exhausts flue gases. They are not interchangeable, and changing between them is licensed work. Condensing — a high-efficiency gas design that extracts extra heat from exhaust and produces acidic condensate needing a drain.

How to choose

If a quote uses a term you cannot define, ask

A good installer will explain FHR or vent type in a sentence. Vagueness on these terms is a warning sign about the rest of the work.

Do not confuse gallons with capacity

Tank size and first hour rating are different numbers. Buy on FHR.

UEF only compares within a category

A heat pump UEF above 3.0 does not mean it is three times cheaper than gas. Compare estimated operating cost using your own rates.

Vent type is a safety term, not a spec detail

It determines how combustion gases leave the building. Any change to it needs a licensed professional and, in most jurisdictions, an inspection.